Write a part's solution by command

[peterbjohnson]

A draft has question solution q1 --text RANGE and nothing that writes q1.p2.solution; only a spec with a layout that pairs parts with solutions writes a part's solution. The commands cannot reproduce PartPartSolSol or PartSolPartSol, and the agent's fix loop cannot answer q1.p2 has no solution where the solution is on the sheet under the part.

Add in2lambda draft part solution PART --text RANGE | --literal TEXT [--by NAME], where PART is q1.p2, writing q1.p2.solution at layer 3 or 4 as question solution does, refusing a PART that is not a part of the draft. Log and replay it like every other command; the report's no-solution check already reads qN.pM.solution. Add it to the command table in docs/source/drafts.md and the CLI reference, and to the agent-facing command list in in2lambda's README if there is one. A test in tests/test_draft.py writes a part solution by range and replays. Done when a draft built by commands alone can hold the PartPartSolSol example's four solutions.

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Workbench ticket t53.

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (1806275) to head (0fc52d5).
⚠️ Report is 15 commits behind head on dev.
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Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #78      +/-   ##
==========================================
+ Coverage   96.61%   96.68%   +0.06%     
==========================================
  Files          24       24              
  Lines        2069     2109      +40     
==========================================
+ Hits         1999     2039      +40     
  Misses         70       70              

Flag	Coverage Δ
unittests	96.68% <100.00%> (+0.06%)	⬆️

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